Photomicrograph of a growing bone spicule stained with Mallory-Azan. Osteocytes are embedded within the bone matrix of the spicule, which is stained dark blue. These cells are metabolically active, laying down the unmineralized bone matrix (osteoid). A number of osteoblasts are aligned on the right side of the spicule. Between these cells and the calcified bone spicule is a thin, light-blue-stained layer of osteoid. This is the uncalcified matrix material produced by the osteoblasts. One of the cells (arrow) has virtually surrounded itself by its osteoid product; thus it can now be called an osteocyte. On the left side of the spicule, the nongrowing part, are inactive osteoblasts. The cells exhibit flattened nuclei and attenuated cytoplasm. x550.
and bone remodeling. As osteoid deposition occurs, the osteoblast is eventually surrounded by osteoid matrix and then becomes an osteocyte.
Osteoblast processes communicate with other osteoblasts and with osteocytes by gap junctions
At the electron microscope level, osteoblasts exhibit thin cytoplasmic processes that penetrate the adjacent osteoid produced by the cell and are joined to similar processes of adjacent osteocytes by gap junctions. This early establish ment of junctions between an osteoblast and adjacent osteocytes (as well as between adjacent osteoblasts) allows neighboring cells within the bone tissue to communicate.
The osteoblast cytoplasm is characterized by abundant rER and free ribosomes (Fig. 8.9). These features are consistent with its basophilia observed in the light microscope as well as with its role in the production of collagen and proteoglycans for the extracellular matrix. The Golgi apparatus and surrounding regions of the cytoplasm contain numerous vesicles with a flocculent content that is presumed to consist of matrix precursors. These vesicles are the PAS-staining
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